In recent years, a reduction in fuel consumption of automobiles has been increasingly desired along with a demand for energy conservation. In order to deal with such a demand, a further reduction in rolling resistance of tires has been desired. The rolling resistance of tires may be reduced by optimizing the tire structure. In general, the rolling resistance has been reduced by utilizing a rubber composition that exhibits low heat build-up.
In order to obtain such a rubber composition that exhibits low heat build-up, various modified rubbers have been developed for a rubber composition that contains silica or carbon black as a filler. For example, a method of modifying a polymerization active end of a conjugated diene polymer obtained by anionic polymerization using an organolithium compound with an alkoxysilane derivative having a functional group that interacts with a filler has been proposed.
However, most of these methods are applied to a polymer of which the polymer end exhibits living properties. Specifically, only a small number of methods that modify cis-1,4-polybutadiene that is particularly important for a tire sidewall rubber or a tire tread rubber have been proposed. Moreover, the modification effect on a rubber composition that contains carbon black or silica is not necessarily sufficient. In particular, a modification effect on a carbon black-containing rubber has not been obtained with regard to cis-1,4-polybutadiene.
In order to solve the above problems, a method that produces a conjugated diene polymer in which carbon black is easily dispersed by reacting a specific functional group-containing compound (modifier) having a nitrogen atom in the molecule with a conjugated diene polymer (see Patent Document 1), and a method that modifies or couples a conjugated diene polymer with at least one compound selected from the group consisting of a quinone compound, a thiazole compound, a sulfeneamide compound, a dithiocarbamate compound, a thiuram compound, and a thioimide compound to produce a modified conjugated diene polymer (see Patent Document 2), have been proposed.
A method that reacts an active end of a conjugated diene polymer having a high cis content obtained using a rare earth catalyst with a functional group-containing alkoxysilane derivative that interacts with a filler to obtain an end-modified conjugated diene polymer, and a method that adds a condensation accelerator to the reaction system when performing alkoxysilane modification have also been proposed (see Patent Documents 3 and 4).    Patent Document 1: JP-A-2001-139634    Patent Document 2: JP-A-2002-30110    Patent Document 3: WO 03/046020    Patent Document 4: JP-A-2005-8870